End of the road for this pack

After two years and ~2000 mi of daily riding (about a 3 Mi round trip, normally) and daily charging, this pack is toast:

Surprisingly, as this photo was taken, it was still at 51.3V after a 2 Mi. trip! (THAT won't last!)

I generally parallel charge the four batteries with a Vector 12V charger at the 2.5A rate, meaning each cell is charged around .62A rate. Recently, the charger has not been shutting off at its customary 14.7V point. (Amp and voltages are as reported by the Vector charger, not a calibrated VOM.) A slip in charging procedure last Monday left the charger on at the 6.5 A rate all day after the pack had been charging through the night at 2.5A. I came home to a warped and hot battery #1.

Surprisingly, the pack still seemed to be a full voltage, and individual battery voltages were all in line. I did a 4 Mi test drive and the pack behaved well, with no excessive voltage sag. I used the stock series charger to recharge, pulling it off the pack manually after replacing 125% of the used power.

Last night, I again put it on the series charger, but forgot to remove it. In the morning, the charger was still working at its full capacity. The DrainBrain (Cycle Analyst ancestor) reported that 18AH had been pumped in after draining 4 AH. The results are as pictured. Sigh. I'm pretty satisfied with their life, though I'd like to know what else I could have done to protect the pack. Perhaps a thermal shutoff could be retrofitted. Any suggested circuits?

Here's how they started:

Two questions:

Any suggested circuits for a thermal shutoff?

What was the failure mechanism in the batteries that kept the batteries from getting to their full voltage, but where they were all above 13V on charge? (I think a shorted cell would have left one battery at ~10V)

cool photo, but sorry it happened. It's amazing they are still working so well. Some of the members here use a timer device to shut down power to the charger after a set time. I noticed you mentioned twice that you forgot to stop the charging and the timer could help there. I don't use one, but if you do a search of the site, you should find some suggestions.

What was the failure mechanism in the batteries that kept the batteries from getting to their full voltage, but where they were all above 13V on charge? (I think a shorted cell would have left one battery at ~10V)

Hi Mark,

It's almost always cheaper to go LiFePO4 than to buy a replacement lead acid battery, and implement temp compensated charging.

the Failure mode goes like this:
There is a secondary reaction inside the battery, that uses current to convert water into hydrogen and oxygen when the cell voltage goes above ~2.3v.
There is a internal catalyst that converts the hydrogen and oxygen back to water, and gives off heat.

The exact voltage that starts changes with the concentration of a few dissolved Ions.
The amount of current absorbed is a function of both temperature and how far above the starting voltage the cell is actually operating at (more temp or more voltage means more current).

Now your charger's profile is usually either cc-cv-cv or cc-cv-cc
Its the final stage we are interested in here.

basically your charger shuts down when the charge voltage and current reaches some value (usually 57.6v 1.5A for 40Ah batteries).

so your charger either holds 57.6v until current falls to 1.5A (cc-cv-cv)
or it charges at 1.5A until voltage rises to 57.6v (cc-cv-cc)

Thermal runaway is where the secondary reaction consumes so much current that the termination condition is never met.

IBscootin, you are right about the timer as an ultimate backup. I had gotten to trust the voltage cutoff of the Vector charger over the two years. Even towards the end, when it wasn't cutting off, the charge rate was so mild there was little excess energy to dissipate.

Antiscab: two reactions. You say,

It's almost always cheaper to go LiFePO4 than to buy a replacement lead acid battery, and implement temp compensated charging.

How can this be? The replacement SLA pack that GreenSaver is already shipping me is $236.00. A backup thermal shutoff circuit couldn't cost more than $50.00. Those costs are dwarfed by the cost of even a 10AH 48V LiFe pack (remembering that it also would require a BMS and a new charger). A brief Google search shows a "Headway" pack, BMS, and charger for $684. (Is this a reputable company? "Ping" is also mentioned some times, but the idea of ordering something individually from what I assume is a Chinese supplier gives me the willies.) Granted, the nominal number of cycles of a LiFe battery is wonderful, but I've seen too many cases of premature failure to believe the numbers outside of a rigorously controlled electrical environment such as a Chevy Volt.

On the other hand, your description of the failure mode makes a lot of sense. I suppose that the Vector's cut-off voltage was just enough too high so that gassing slowly exceeded that capacity of the internal catalyst was slightly decreased.

How can this be? The replacement SLA pack that GreenSaver is already shipping me is $236.00. A backup thermal shutoff circuit couldn't cost more than $50.00. Those costs are dwarfed by the cost of even a 10AH 48V LiFe pack (remembering that it also would require a BMS and a new charger). A brief Google search shows a "Headway" pack, BMS, and charger for $684.

Hi Mark,

Generally when the battery starts heating up, its already overcharging.

If you set the temp cutoff conservatively enough to prevent excessive overcharge, you will likely encounter nuisance tripping.

If you set it too high, than the battery will already be damaged by the time the thermal cutoff shuts down.

I have found a timer to be more useful, but I would still suggest having a go using a thermal cut-off anyway since you already have the replacement batteries on the way.

IMO, changing the max charge voltage in both the cv and terminating values based on temperature is a better way to go. It's to difficult to be worthwhile usually to do it to a charger not designed for it.
None of the cheap commodity chargers are designed for it.

IIRC, TC Charger's support temperature compensation, but their smallest one costs ~$450 in single unit quantities (although it is 1500W).
I'm not aware of any smaller units.

Headway is a good LiFePO4 brand, continuous discharge rate rating is quite high (much higher than large format prismatics).

Unless you need the discharge rate, you would be better of going with a larger cheaper battery.

Whats the peak and continuous amps your scooter draws?
How many Ah do you use in a day?

a 48v 20Ah LiFePO4 battery and charger can be had for $500, but if your scooter draws more than 30A continuous, it will not be large enough.

"How can this be? The replacement SLA pack that GreenSaver is already shipping me is $236.00. A backup thermal shutoff circuit couldn't cost more than $50.00. Those costs are dwarfed by the cost of even a 10AH 48V LiFe pack (remembering that it also would require a BMS and a new charger). A brief Google search shows a "Headway" pack, BMS, and charger for $684. (Is this a reputable company? "Ping" is also mentioned some times, but the idea of ordering something individually from what I assume is a Chinese supplier gives me the willies.) "

It's my understanding that Greensaver batteries are a Chinese product from a Chinese company. Don't panic, but don't think you aren't buying a Chinese battery pack...

Generally when the battery starts heating up, its already overcharging.

That makes sense, given that the thermometer for such a rig would be outside the case. Still, do you know of a circuit for a thermal shutoff or a device that would act as a thermal circuit breaker? Since the literal meltdown, I've been using a timer to nurse the sick pack until the replacements arrive. I will probably continue to use a timer as backup in the future with the new batteries.

About the LiFePO4 question, I'll revisit it when this pack dies in a few years. I think a 48V 15AH pack would be adequate. The max measured current draw has been 950 watts.

LeftieBiker, you say;

It's my understanding that Greensaver batteries are a Chinese product from a Chinese company. Don't panic, but don't think you aren't buying a Chinese battery pack...

A) I misspoke, it's not Greemsaver; it's actually ordered from greenmax​distributor​s.com, the folks that handle parts for X-Treme scooters. I've had good experiences with them in the past. Their stock is in the US. I'm sure the batteries are Chinese (Come on, look at the photo of the battery side above: a "shortage battery"????), but I have seen how these batteries hold up, and they're pretty good.